Ham Radio

[Nowhere-Man]

I just had one of my cerebral metltdowns over diodes and also AVG diodes. Maybe this happened because I see far more triodes and pentodes used as detectors. Still these American HAM circuits (vintage) are a bit striking. The first IF transformer supplies two diodes. The detector had the AF taken from a cathode circuit (resistor and capacitor. The second diode had the same feed as the first from the IF can but was only used for AVG. So it was resistor biased. I magine making that circuit with the two diodes. Actually the schematics now seem fairly clear as I got more used to the American diagrams. Neither were some of them overly complicated. I love the fact you don't have to string dialchord drive mechanisms.

[pwdrive]

Apart from the crystal set, diode detectors were mainly to be found in superhet receivers and as the mixer in direct conversion home brew Ham receivers. Triode and pentode detectors were mainly used in regenerative receivers.

Sometimes in a superhet the mixer was referred to as the 1st detector.

Lawrence.

[Terry]

I assume that by AVG* you mean AVC (Automatic Volume Control) but it will avoid confusion later if you try to think of this as AGC (Automatic Gain Control) as this is the term in use since television has been around because it covers both sound and vision channels - there is no actual difference apart from the name change.

* Or is AVG a strange American variant of AGC?

[Nowhere-Man]

I think you refer to the DDT detectors in the superhets, Lawrence. I know it amounts to the same thing but the schematic I have shows two diode valves apart. What I did pick up on was the first diode (the RF detector) shows the load resistor with the + side and cathode to chassis. That is, signal passes through diode to the AF load resistor at cathode with it's capacitor but the chassis symbol there where it was I found odd. The AVC diode was fed the signal the same way.
AVC is used in America too so AVG must have been my own conception.
Sometimes I find simple things that had slipped me by. Last week was the first time I started to understand bandwidths. In the days of HAM I guess this was taught in classes over term times.
Today I slept late as very tired from working on the boat yesterday. Was in the engine bilge.

[pwdrive]

Predominantly diode detectors in superhets, the triode section of a DDT in a superhet being used as the 1st audio amplifier.

Lawrence.

[Nowhere-Man]

Tonight I was looking at the Hartley oscillator circuit. To summarise what I learned the cathode in the normal circuit is at - and tapped about the centre of the inductor (in parallel with its capacitor). Then each opposite of the inductor connect to anode and grid, through blocking capacitors. The tap is said to determine the feedback. ARRL is often a bit lacking in down-to-earth explanations. I sometimes wonder if the Radio Corps students back then genuinely followed what was taught in class. It took me a lot headaches to understand that feedback can simply detract from or add to the cathode grid signal.
My hope is at some point I may be able to build a receiver. I bet that would be quite a kick..

[pwdrive]

You could try building a simple Ham receiver by using a regenerative design, the Hartley osc is well suited to that, one end of the grid tank coil is connected to HT-ve (ground) the other end is connected to the grid via the usual grid leak arrangement and the cathode is connected to a tap on the coil, the tap should be towards the ground end, experimentation is sometimes required for the tap position in order to get the best results, the screen grid would be connected to a variable potential divider (pot) varying the screen grid voltage varies the amount of regenerative feedback, audio output is via the anode load as usual, for AM reception then just below the point of oscillation is best, for CW or SSB reception then just above the point of oscillation is best, lots of designs on the web, an excellent set up so far as regens go is the Regenerodyne type with an RF stage, the tuning rate is constant across any band with that set up, you can either build a VFO for osc injection or alternatively use a an ordinary service type RF signal generator if it has enough output level.

Lawrence.

[Nowhere-Man]

I found this while browsing:
"

Due in part to rising rent in the city, the outlet at 5
S10 Lawrence Expressway chose to close its doors and merge operations with its Oakland location, according to shop employees.The Sunnyvale Ham Radio Outlet signed off for the final time Wednesday after 26 years as a hub for Silicon Valley’s amateur radio community."

[Nowhere-Man]

I was reading ARRL transistor and semiconductor pages. Although it is great for valves, the transistor explanations are choppy and not very clear. For example, this business about the alpha. ARRL didn't even use the term, just a symbol. Then, out of nowhere, like a rabbit from a magician's hat it quoted 1 - 0.98 = 0.02 to give an example of the short circuit current transfer ratio. Baffled, I searched on Google and found a simple description. Isn't it the case that often the person writing the book should never assume the reader has similar knowledge? I still don't know what the "short circuit" term means. Collector to ground maybe?
The important punch line I understand. Namely current is amplified.
Transistors in ARRL are only a tiny percentage and it seems nobody imagined they would replace valves.

[Terry]

A lot of material in the early days of transistors was like that - see my earlier post about holes

Endless descriptions of what went on inside the germanium structure rather than how to use such devices were not helpful with comprehension.

Does a description of how a valve works require a detailed description of the chemical composition of the cathode coating? No, of course not! You want to know how it works as viewed from the outside, not how to manufacture your own valves!